Process of developing photographic images, with conversion prom negative to positive



Patented .luly 2, 1935 PATENT OFFICE PROCESS OF IMAGES,

DEVELOPING PHOTOGRAPHIC WITH CONVERSION FROM NEGATIVE TO POSITIVE Walter Kleist, Munich, Germany, assignor to the firm Otto Perutz, Trockenplatteniabrik Munchen G. in. b. H.,

Munich, Germany No Drawing. Application March 1'1, 1934, Serial No. 716,175. In Germany March 25, 1933 1 Claim.

5 then washing with water and dissolving out the negative silver, and finally reducing the unconverted silver bromide, after a brief treatment in a clearing bath and exposure. The composition of the developer first used depends on the kind of emulsion and the nature of the positive which is to be produced, but this developer is always alkaline, sometimes strongly alkaline, whereas the silver solvent bath, consisting 101' example of permanganate of potassium and sulphuric acid, is strongly acid. The second deyeloper is alkaline again. The alternation of alkaline and acid treatments produces structural changes which are damaging to the appearance of the picture in the gelatine. It has been sought to ,obtain a better result by using for the second developer substances which reduce the silver bromide in the acid state, but the acid baths used for the purpose are unstable, their life in some cases being limited to a few hours. Hardening by means of sulphates of aluminium and chromium is not permanent, and the efiect of the alkali'is reduced by the second developer. Another proposal which has been made is to introduce a hardening bath after the first developing treatment, using for example tormaldehyde, but this. hardens the gelatine so much that a much longer treatment with water is required, as otherwise yellow clouding is liable to result from the second developing treatment. A similar disadvantage arises ii formaldehyde is added to the first developer,

and this method has the secondary disadvantage that the developer must be put up for sale in bottles.

According to my invention the difiicult ies and disadvantages referred to are avoided by adding hexamethylenetetramine to the first developer. This is a crystalline powder, which is perfectly stable in the chemical sense and is easy to handle.

Whereas with the usual film emulsion flotationand injury occur at about 32 C. an increase to about 42 C. is obtained by using our developer. The mild but quite adequate hardening efiected does not involve any substantial increase in the periods required for soaking in water.

Example.-From 5 to 10 grams of hexamethylenetetramine are added, per litre, to a developer appropriate to a silver bromide emulsion. 20

The addition of hex-amethylenetetramine to developers for developing photographic negatives, for hardening purposes, is already known, but only in connection with carrying out the developing process with heat, say at 30 or C.

What I claim as my invention is:

The process of developing photographic images, with conversion from negative to positive comprising the following steps: developing the image in an alkaline developer'containing hexso amethylenetetramine, washing the developed] image in water, dissolving out the negative thus" produced, and finally, after exposure, reducing the unconverted silver-bromide in a second alkaline developer.

WALTER KLEIST. 

